Avoid extra frame drawn around color wheel in menus

Not needed, and looks very ugly with Qt5 styling

src/gui/qgscolorwidgets.cpp

@@ -405,12 +405,6 @@ void QgsColorWheel::paintEvent( QPaintEvent *event )
   Q_UNUSED( event );
   QPainter painter( this );
 
-  //draw a frame
-  QStyleOptionFrame option = QStyleOptionFrame();
-  option.initFrom( this );
-  option.state = this->hasFocus() ? QStyle::State_Active : QStyle::State_None;
-  style()->drawPrimitive( QStyle::PE_Frame, &option, &painter );
-
   if ( !mWidgetImage || !mWheelImage || !mTriangleImage )
   {
     createImages( size() );